Bear Trap

Yes. Bear trap is not dealing spell damage, and is not affected by increases to such.

Spell damage does seem to increase bear trap damage significantly.

If you are referring to the character screen, this is incorrect. Because the character screen uses the same set of stat to describe al the skills, it currently can't always show the correct values - it assumes all non-attack skills deal spell damage show shows spell damage stats. Bear Trap does not deal spell damage, however, so those stats have no meaning to the actual skill. The correct stats for damage should be shown on the skill popup.

I just did a quick test (not very scientific, but it gave results nonetheless) regarding this last point. Having increased duration linked to this skill only made the trap persist longer, and did not hold the target any longer than without it. Isn't that supposed to make the target be trapped for a longer duration?

How did you test this? the base duration is different each time as it's based on the monster's life compared to damage done, and the damage varies over a large range, making successive trappings not directly comparable. The relevant stats are definitely applied in the code.

The character sheet shows the same stat set for all skills, as such it can't handle skills that are technically spells (due to not being attacks) but deal non-spell damage.

The actual skill popup will show the correct damage, because that's different and customised to each skill, so knows which stats to look at.

this is way too confusing. When I put on spell damage wands modifiers it clearly affected bear trap skill popup. same as spell crit chance/spell dmg multiplier on the character sheet.

The character sheet is not the skill popup. The skill popup is correct, and does not show increase from spell damage. The character panel does, and is wrong.

If I give my character 99999% more spell damage, that applies to flamme totem and fireball, but the skill popup for Beartrap does not change. If you're seeing it change only because of spell damage bonuses, then that's a bug, as they do not apply, and I need to find out how to replicate it, so that I can fix it.